CH4 Flow Research Articles

Enzyme activities of rhizosphere soil with different root diameters had varied responses to N deposition in Pinus tabuliformis forest

Root system exhibits hierarchical traits in physiology and exudation, and responds variably to environmental changes. However, it remains unclear if these phenomena result in corresponding alterations in the microbial characteristics of rhizosphere soil and diverse responses to nitrogen (N) deposition. This study evaluated microbial biomass, gas flow, and enzyme activity in rhizosphere soil of three root diameter classes (R1, <0.5 mm; R2, 0.5–1.0 mm; R3, 1.0–2.0 mm) of P. tabuliformis, and their responses to N application (0, 3, 6, 9 g N m−2 y−1, corresponding to CN, LN, MN, and HN) in a P. tabuliformis forest. Rhizosphere soil of R1 exhibited significantly higher microbial biomass N (MBN) content, enzyme activities, CO2 and N2O flow rates than those of R2 and R3 (P < 0.05). Soil microbial biomass carbon (MBC) and phosphorus (MBP) contents, N2O and CH4 flow rates significantly increased with N applications. While soil MBN content increased with the LN treatment and decreased with the HN treatment, the largest value was observed in the LN treatment (84.89 mg kg−1). N application promoted Alkaline phosphatase (ALP) and Alanine transaminase (ALT) activities in R1 rhizosphere soil while decreasing their activities in R2 and R3 rhizosphere soils. Moreover, β-glucosidase (BG), N-acetyl-β-Glucosaminidase (NAG), β-xylosidase (XYL), and β-D-cellulosidase (CBH) activities in R1 rhizosphere soil were inhibited, while their activities in R2 and R3 rhizosphere soils were promoted by N application. N application directly promoted microbial biomass then inhibited enzyme activity. Besides, nutrient content of rhizosphere soil decreased with root diameters, which indirectly reducing enzyme activity and microbial biomass. N application directly affected soil CH4 flow rate, while N2O flow rate was indirectly promoted by N application through increasing microbial biomass. CO2 and CH4 flow rates were inhibited by N application through decreasing enzyme activity. Root diameter slowed down the CO2 and CH4 flow rates by reducing soil nutrient content and enzyme activity. In conclusion, N deposition has varied effects on microbial characteristics in rhizosphere soil of different diameter roots, which are relevant to the corresponding changes in root system and soil properties.

Experimental study of coal rank effect on carbon dioxide injection to enhance CBM recovery

Coal seam gas extraction is an important part of green mining. At the same time gas injection for enhanced coalbed methane recovery (ECBM) has been proposed as an environmentally friendly, low-carbon and effective technology to increase methane production. To study the influence of coal rank on carbon dioxide (CO2)-ECBM efficiency, lignite, bituminous coal, and anthracite were tested using the Gas Flow and Displacement Testing Apparatus (GFDTA). Data indicated that: (1) in lignite and anthracite, outflow CO2 and CH4 fractions changed at higher rates as compared with bituminous coal. At the end of the experiments, lignite and anthracite displayed CH4 outflow fractions of 0.19% and 7.73%, respectively; (2) at the initial stage of the experiments, permeability rapidly decreased. Later, a fast increase was followed by a stable phase. In lignite and anthracite, the increment in permeability during the rapid increase stage was significantly larger than that of bituminous coal. (3) The CH4 flow rate of the three coal ranks showed a decreasing trend. Before the breakthrough, CH4 flow rate decreased rapidly and slowed down. After the breakthrough, CO2 flow displayed a linear increasing trend and later reached stability. (4) In general, CH4 production increased with the increase in coal rank. The variation of CH4 production is manifested in two phases with different dominant factors. In the first stage, CH4 production was mainly affected by permeability, and followed the order lignite > anthracite > bituminous. In the second stage, CH4 production was mainly controlled by the displacement effect, following the order anthracite > bituminous coal > lignite.

Experimental study on soot formation and primary particle size in oxy-combustion ethylene diffusion flames under CO2 substitution for N2

This paper reports an experimental study on the effects of O2 concentration and CO2 concentration on soot volume fraction and primary particle size of flame centre in ethylene laminar co-diffusion flame. The incandescent light was induced by a two-color band method and laser-excited by wavelength 1064 nm. Previous studies have shown that the oxygen concentration affects the soot volume fraction in the flame. However, the effects of O2 concentration on the primary particle size under CO2 atmosphere are unclear and need to be further discussed. Seven flames were optically detected, in which the oxygen concentration of the accompanying gas varied from 21% to 50% by volume. One primary flame was burned in co-flowing ethylene/air. Six ethylene flames were burned in an oxygen-rich O2/N2 or O2/CO2 atmosphere with an oxygen mole fraction from 30% to 50%. The C2H4 flow rate remained constant in all operating conditions. The results show that the SVF observed by the LII method and spectroscopy in the oxygen-enriched flame are in good agreement. Furthermore, the soot's primary particle diameter measured by the time-resolved LII method is in good agreement with the transmission electron microscope image analysis.With the increase of oxygen concentration, the flame height becomes shorter, the luminosity becomes brighter, and the soot yield increases. Compared with an N2 atmosphere with the same oxygen concentration, the soot yield in a CO2 atmosphere was significantly inhibited. In all the flames studied, the SVF along the flame axis showed a similar distribution: it first increased to the maximum value and then decreased rapidly until the flame tip. The maximum SVF on the axis appears between 0.64 and 0.78 based on the normalized height of the visible flame. In all ethylene/(O2/N2) flames, the maximum primary particle diameter and average primary particle diameter increase with oxygen content. For ethylene/(O2/CO2) flame, the maximum diameter of primary particles decreases with the increased oxygen content. The uncertainty of primary diameter is mainly caused by the thermal adaptation coefficient, the lower LII signal of smaller particles and the uncertainty of local flame temperature.

Comparative study of titanium carbide films deposited by plasma-enhanced and conventional magnetron sputtering at various methane flow rates

Titanium carbide (TiCx) films were deposited by reactively sputtering Ti target with varied methane (CH4) flow rates using an unbalanced magnetron sputtering equipment operating in conventional (CMS) and plasma-enhanced (PEMS) mode. The microstructure and properties of the TiCx films were mainly scrutinized with X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, nano indentation, and ball-on-disk tribo-test. As the CH4 flow rate is increased from 4 to 16 sccm, x in the TiCx films deposited by PEMS and CMS is gradually increased from 0.28 to 0.96 and 0.28 to 0.73, respectively; and the concentration of TiC phase in the films rises. A densified microstructure with strong preferred growth is observed in the PEMS-deposited films, while a loose columnar microstructure with almost random growth exists in the CMS-deposited films. When the CH4 flow rate is increased, the hardness of the films deposited by PEMS is firstly increased and then decreased, and reaches a maximum value of 29.6 GPa; but that of CMS-TiCx film is fluctuated below 10.6 GPa. The friction coefficient of TiCx-coated samples against pure aluminum is gradually increased with increasing CH4 flow rate from 4 to 14 sccm, accompanied by a reduction of the aluminum adhesion. The wear rate of the TiCx-coated samples as a function of the CH4 flow rate is firstly decreased and then increased. The lowest wear rate of the PEMS-prepared samples is 7.2 × 10−16 m3/(N·m) obtained at a CH4 flow rate of 14 sccm, while that of the CMS-prepared samples is 9.3 × 10−15 m3/(N·m) obtained at 12 sccm. Therefore, PEMS is a promising method to prepare TiCx films with a good combination of the mechanical properties and wear-resistance against pure aluminum.

Effect of CH4 Flow Rate on the Tribological Behaviors of TiCN Films against Si3N4 Ceramic and Steel Ball

Control of the structural, mechanical and tribological properties of TiCN films play an important role in its numerous applications, including the cutting tools, mechanical components, aeronautical and biomedical engineering industries. Direct-current magnetron sputtering (DCMS) system was applied to deposit TiCN films onto n-type silicon (100) at room temperature. The Ti-TiN interlayer was used to enhance the adhesive strength between the coating and the substrate. The composition and microstructure of the TiCN films were studied using X-ray photoelectron spectroscopy (XPS) and field-emitted scanning electron microscopy (FESEM). The mechanical properties of the films as a function of methane (CH4) flow ratio were then characterized using nano-indentation measurements. The tribological behavior of TiCN films was investigated by UMT-2MT tribometer against a Si3N4 ceramic and AISI52100 steel ball. After the tribological tests, the wear rate of the films was obtained by the 3D surface profiler and the component content of wear debris was evaluated by energy dispersive X-ray spectroscopy (EDS). The results show that the tribological properties of TiCN films are a function of CH4 flow rates. The film obtained at a 10 sccm CH4 flow rate possesses a minimum average COF value of 0.1964 and reaches 72,000 cycles against a Si3N4 ball over the test duration. Furthermore, the wear rate was only 2.076 × 10−6 mm3/N·m. Furthermore, the TiCN films exhibited longer lifespan against the Si3N4 ball than against the steel ball under the normal load of 1 N, indicating that the TiCN films present better lubricative properties when against low-hardness counterparts than high-hardness counterparts.

Epitaxial growth of TiC on (0001) 4H-SiC substrate by reactive sputtering

Heteroepitaxial TiC films were grown on semi-insulating (0001) 4H-SiC substrates at high temperature by using direct current (DC) magnetron reactive sputtering. The effect of the CH4 flux ratio in Ar on the crystalline quality of the films was investigated. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the deposited TiC films are heteroepitaxially grown on 4H-SiC substrates with the heteroepitaxial relationship between TiC and 4H-SiC in (111)TiC // (0001)4H-SiC and [11¯0]TiC // [112¯0]4H-SiC. The full width at half-maximum (FWHM) of X-ray rocking curve (XRC) of (111)TiC reflection can reach a minimum value of 0.039° and the resistivity of TiC film can be ∼ 80 μΩ⋅cm for deposition at 700 °C with 4% CH4 flow ratio. X-ray photoelectron spectroscopy (XPS) shows that the composition of TiC film is nearly in stoichiometry for 4% CH4 flow ratio. From the TEM results, it is observed that the TiC film is divided into a strained layer and a relaxed layer when the film thickness reaches a critical value over about 70 nm.

Effects of pulsed gliding arc plasma on non-premixed CH4/CO2– air flame stability

Effects of carbon dioxide (CO2) addition and pulsed gliding arc (PGA) plasma on methane (CH4) flame stability and pollutant emissions are investigated in this paper. Two main parts are reported: the effects of CO2 addition to the fuel and the impact of PGA plasma on the characteristics of non-premixed CH4/CO2-air flames in a coaxial swirl burner. The burner consists of 2 concentric tubes, the central tube supplies the CH4 flow and the periphery one supplies the airflow. In the central tube, a metal rod is used as a cathode. To produce the PGA plasma, two other electrodes are symmetrically placed around the central one to obtain two PGA plasma zones in the stabilization area of flame. The combustor is a parallelepiped chamber, with a 0.48 m square cross section and 1 m height equipped with six windows on each side. This work primarily focuses on pollutant emissions (NOx and CO), flame structures, and stability. OH* chemiluminescence measurements are used to describe the structure and stability of the flame providing information on the flame lengths and lift-off heights. The lift-off heights and the flame lengths are determined as a function of the CO2 concentrations, ranging from 0% to 50% in the fuel, at a fixed swirl number and equivalence ratio. The results show that CO2 addition to the fuel changes significantly the flame shape and its behavior. The flame becomes more unstable at high CO2 percentages. CO2 addition dilutes reactants, reduces NOx production, and increases CO concentrations in the flue gases. To overcome the instabilities problems due to the addition of CO2, the PGA is used and investigated with different parameters of plasma and flame. PGA plasma is generated by a DC pulsed power generator with HV pulse duration of few microseconds. It is observed that the presence of PGA plasma decreases flame instabilities and reduces CO concentration in the flue gases.

Evaluation of process conditions for methane pyrolysis applying the triple thermal plasma system

Among various thermal plasma sources for CH4 pyrolysis, singular direct current (DC) thermal plasma has limited capacity, erosion, and efficiency. However, the triple thermal plasma system has been applied for nanomaterial synthesis and exhibited results overcoming these limitations. This study used the triple thermal plasma system to investigate the most suitable conditions for CH4 pyrolysis. CH4 conversion rate and selectivity of H2 and C2H2 were analyzed by varying the CH4 flow rate and quenching conditions at a fixed power supply of approximately 30 kW, and the specific energy requirement (SER) per 1 kg H2 was compared with that of previous works. The maximum conversion rate was 97% at 50 L/min of CH4, which is approximately 7% higher than earlier studies under conditions with similar process enthalpy. In addition, the conversion of CH4 to C2H2 and further to heavier hydrocarbons proceeded one order of magnitude faster than the reaction time expected by the gas-phase reaction. This result is attributed to the easy penetration of CH4 into the core region with the highest temperature and the strong interaction between the processing gas and graphite surface due to the arrangement of the torches in the triple plasma system. C2H2 selectivity was relatively high, while it was less affected by the increase in the quenching gas than generally expected. This finding was attributed to the naturally fast quenching rate without quenching gas due to the structure expanding from the first to the second graphite. While quenching can enhance selectivity by stabilizing the radicals as intermediates such as H2 or C2H2, it depressed the following reaction with dehydrogenation. Thus, the quenching conditions must be optimized. Finally, we demonstrated that the triple thermal plasma enhanced CH4 pyrolysis regarding H2 production efficiency.

Electrosynthesis of polymer-grade ethylene via acetylene semihydrogenation over undercoordinated Cu nanodots

The removal of acetylene impurities remains important yet challenging to the ethylene downstream industry. Current thermocatalytic semihydrogenation processes require high temperature and excess hydrogen to guarantee complete acetylene conversion. For this reason, renewable electricity-based electrocatalytic semihydrogenation of acetylene over Cu-based catalysts is an attractive route compared to the energy-intensive thermocatalytic processes. However, active Cu electrocatalysts still face competition from side reactions and often require high overpotentials. Here, we present an undercoordinated Cu nanodots catalyst with an onset potential of −0.15 V versus reversible hydrogen electrode that can exclusively convert C2H2 to C2H4 with a maximum Faradaic efficiency of ~95.9% and high intrinsic activity in excess of −450 mA cm−2 under pure C2H2 flow. Subsequently, we successfully demonstrate simulated crude ethylene purification, continuously producing polymer-grade C2H4 with <1 ppm C2H2 for 130 h at a space velocity of 1.35 × 105 ml gcat−1 h−1. Theoretical calculations and in situ spectroscopies reveal a lower energy barrier for acetylene semihydrogenation over undercoordinated Cu sites than nondefective Cu surface, resulting in the excellent C2H2-to-C2H4 catalytic activity of Cu nanodots.

Acidification of slurry to reduce ammonia and methane emissions: Deployment of a retrofittable system in fattening pig barns

Livestock farming, and in particular slurry management, is a major contributor to ammonia (NH3) and methane (CH4) emissions in Europe. Furthermore, reduced NH3 and CH4 emissions are also relevant in licensing procedures and the management of livestock buildings. Therefore, the aim is to keep emissions from the barn as low as possible. Acidification of slurry in the barn can reduce these environmental and climate-relevant emissions by a pH value of 5.5. In this study, an acidification technology was retrofitted in an existing fattening pig barn equipped with a partially slatted floor. The slurry in a compartment with 32 animals was acidified. An identical compartment was used for reference investigations (case-control approach). Several times a week slurry was pumped for acidification in a process tank outside the barn compartment in a central corridor, where sulphuric acid (H2SO4) was added. Then the slurry was pumped back into the barn. In contrast to other systems, where acidified slurry was stored mainly in external storage tanks, in this study the slurry was completely stored in the slurry channels under the slatted floor, during the entire fattening period. The emission mass flow of NH3 and CH4 was measured continuously over three fattening periods, with one period in spring and two periods in summer. On average 17.1 kg H2SO4 (96%) (m³ slurry)−1 were used for acidification during the three fattening periods. NH3 and CH4 emissions were reduced by 39 and 67%, respectively. The hydrogen sulphide (H2S) concentration in the barn air of the acidification compartment was harmlessly low (0.02 ppm). Thus, despite the storage of the acidified slurry in the barn, the system leads to a lower concentration of detrimental gases, which is beneficial for the animals' as well as for the workers’ health. The study shows that it is possible to retrofit acidification technology into existing pig barns. Further investigations shall identify possible measures to reduce the amount of H2SO4 used and thus minimise the sulphur input into the slurry.

The Influence of H Content on the Properties of a-C(W):H Coatings

Diamond-like-carbon “DLC” coatings can be deposited in many different ways, giving a large range of material properties suitable for many different types of applications. Hydrogen content significantly influences the mechanical properties and the tribological behavior of DLC coatings, but its determination requires techniques that are not available in many research centers. Thus, it is important to find alternative indirect techniques, such as Raman spectroscopy or nanoindentation (hardness measurements), which can give comparative and indicative values of the H contents in the coatings, particularly when depositions with a reactive gas flow are being studied. In this work, “DLC” composite coatings with varying H content were deposited via Physical Vapor Deposition (PVD) magnetron sputtering in a reactive atmosphere (Ar + CH4). An Ion Beam Analysis was used to determine the elemental depth profile across the coating thickness (giving both average C:W:H ratios and film density when combined with profilometer measurements of film thickness). The hardness was evaluated with nanoindentation, and a decrease from 16 to 6 GPa (and a decrease in the film density by a factor of two) with an increasing CH4 flow was observed. Then, the hardness and Raman results were correlated with the H content in the coatings, showing that these indirect methods can be used to find if there are variations in the H content with the increase in the CH4 flow. Finally, the adhesion and tribological performance of the coatings were evaluated. No significant differences were found in the adhesion as a function of the H content. The tribological properties presented a slight improving trend with the increase in the H content with a decrease in the wear rate and friction.

Ablation threshold and temperature dependent thermal conductivity of high entropy carbide thin films

High entropy carbides (HECs) are a promising new class of ultra-high temperature ceramics that could provide novel material solutions for leading edges of hypersonic vehicles, which can reach temperatures &gt; 3,500 �C and experience extreme thermal gradients. Although the mechanical and thermal properties of HECs have been studied extensively at room temperature, few works have examined HEC properties at high temperatures or considered these materials� responses to thermal shock. In this work, we measure the thermal conductivity of a five-cation HEC up to 1200 �C. We find that thermal conductivity increases with temperature, consistent with trends demonstrated in single-metal carbides. We also measure thermal conductivity of an HEC deposited with varying CH4 flow rate, and find that although thermal conductivity is reduced when carbon content surpasses stoichiometric concentrations, the films all exhibited the same temperature dependent trends regardless of carbon content. To compare the thermal shock resistance of HECs with a refractory carbide, we conduct pulsed laser ablation measurements to determine the fluence threshold the HECs can withstand before damaging. We find that this metric for the average bond strength trends with the theoretical hardness of the HECs as expected.

Investigation on the adsorption properties and adsorption layer thickness during CH4 flow driven by pressure gradient in nano-slits

In shale gas exploration, gas adsorbed on the surface of porous medium results in a change in pore size, which is closely relevant to permeability, flow rate, and production capacity of shale gas reservoirs, especially for the reservoir containing large numbers of pores and slits. Thus, the present work investigates the adsorption mechanism and adsorption layer thickness during CH4 flow driven by the pressure gradient in nano-slits by using molecular dynamics simulation. Herein, a slit-pore model in terms of gas storage and grapheme pore is developed, implemented, and verified. The effects of the pressure, temperature, pressure gradient, and pore size on adsorption properties and adsorption layer thickness of CH4 are also examined. Results show that the relative adsorption capacity is positively correlated with the pressure gradient and pore size and negatively correlated with the system pressure, whereas unaffected by temperature. Moreover, the adsorption layer thickness decreases with the pressure and is almost unaffected by the pore size under the small pore size, whereas increasing with the pressure gradient and temperature. The descending order of sensibility to the adsorption layer thickness is temperature, pressure gradient, pore size, and system pressure. Hence, based on those findings, a new formula for calculating the adsorption layer thickness is proposed for the quantitative determination of the effective pore size of porous medium when gas flows in slits, thereby contributing to shale gas high-efficient exploration.

Structural, mechanical, and tribological properties of CrCN coatings obtained by cathodic arc physical vapour deposition technology at different CH4/N2 gas ratio

Chromium carbonitride coatings were deposited by cathodic arc physical vapor deposition technology at a temperature of 300 °C, as were used the reactive gasses CH4 and N2. The structural analysis of the CrN coating showed a polycrystalline structure with mixed CrN and Cr2N phases. All studied coatings, including the CrC exhibits fcc structure. The phases were confirmed by X-ray photoelectron spectroscopy measurements where a surface oxidation was also detected. The increase of the CH4 gas flow during the deposition process leads to a parabolic trend with the highest hardness of 33.5 GPa for the coating deposited at CH4 / N2 = 0.53. At the same time the lowest coefficient of friction for both counterparts Al2O3 and ZrO2 (0.28 and 0.26, respectively) were measured at CH4 / N2 = 1.86. The tribological tests reveal that the wear of the coatings increases with an increasing CH4 flow rate, whereas the coefficient of friction decreases. This observed contradiction is explained by a phenomenon described as the effect of Rebinder.

Effect of C2H2 flow rate and a Ti/TiN/TiCN interlayer on the structure, mechanical and tribological properties of a-C:H films deposited using a hybrid PVD/PECVD process with an anode-layer ion source

Diamond-like carbon (DLC) films were deposited onto 316L stainless steel using the hybrid physical vapor deposition (PVD)/plasma-enhanced chemical vapor deposition (PECVD) process with an auxiliary anode-layer ion source at C2H2 flow rates of 40–100 sccm to enhance mechanical and tribological properties of a-C:H films. The films were characterized using transmission electron microscope (TEM), field emission scanning electron microscopy (FESEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), nanoindentation and ball-on-disk rubbing tribometry. The hardness, tribological properties and interfacial bonding were significantly improved in the presence of the Ti/TiN/TiCN interlayer deposited by high-power impulse magnetron sputtering (HiPIMS). Results showed that a-C:H films were uniform and dense, the sp3 C content increased with an increase in the C2H2 flow rate from 40 to 80 sccm, and decreased when it was further increased to 100 sccm. The a-C:H film with the interlayer prepared at 80 sccm C2H2 exhibited the highest hardness (27.2 ± 0.5 GPa), H/E (0.137), and H3/E2 (0.510). Moreover, friction-wear tests demonstrated that the incorporation of the Ti/TiN/TiCN layer increased the wear resistance of the a-C:H film, achieving the lowest coefficient of friction (CoF, 0.148), wear rate (1.08 × 10−9 mm3 N−1 mm−1) and width of the wear track (177 μm) at 80 sccm C2H2.

The simultaneous synthesis of carbon dots and carbon spheres with tunable sizes using a vertical chemical vapour deposition method

Herein we report on the direct synthesis of solid hydrophobic carbon dots (CDs) and simultaneously carbon spheres (CSs), using a vertical chemical vapour deposition (CVD) reactor. The HRTEM data indicated that the CDs and CSs originated from different carbon building blocks indicated that the CDs and CSs originated from different carbon building blocks. The CDs were obtained by reacting acetylene (C2H2) and Ar mixtures at high flow rates (&gt;500 sccm) and high temperatures (800-1000 °C). TEM studies indicated that the CDs produced were graphene-like quantum dots that increased in size from c. 3 nm to 8 nm as the Ar flow rate (constant C2H2 flow rate) was decreased, while small solid CSs (c. 100 nm) were also synthesised in the process. The CSs had a typical spherical layered structure with no graphitic core. A mechanism to rationalise the observation that the CDs and CSs grew simultaneously by different pathways with no formation of intermediate-sized particles is given.

Optimization of sputter-deposited corrosion resistant a-C(:H) films with excellent tribological performance by control of methane flow rate

To meet the lubrication demand in the maritime sector, hydrogenated amorphous carbon (a-C:H) films with low friction coefficient and excellent corrosion resistance were deposited at different methane flow rates by control sputtering technique. Studies have shown that the increment of CH4 flow rate raised the growth rate and compactness of a-C(:H) films while decreased the hardness, the adhesion force was enhanced. The film had the best tribological performance at 5 sccm CH4 gas flow rate, with a friction coefficient of about 0.1 (in air) / 0.04 (in salt solution) and a wear rate of 1.0 × 10−9 mm3/Nm / 6.2 × 10−8 mm3/Nm. With increasing CH4 flow rate, the corrosion resistance of the films improved. And 10 sccm was the optimal for the comprehensive performance of the film.

Study on CO2 based thermal plasma torch and its effective utilization for material processing in atmospheric pressure

Reuse of greenhouse gases is one of the promising approaches towards controlling global warming to some extent. As a part of that, an attempt has been made to utilize CO2 gas as one of the primary gases to form a thermal plasma jet for material processing. For that purpose, a unique direct current (DC) non-transferred arc thermal plasma torch was designed and its current-voltage characteristics and electro-thermal efficiency with different discharge currents and gas compositions were studied. The torch is made of rod type cathode and a nozzle type anode, which can produce a stable thermal plasma jet under different gas compositions. CH4 was used as a secondary mixing gas in an Ar-CO2 gas composition to produce a carbon-rich thermal plasma with high electro-thermal efficiency. The maximum electro-thermal efficiency of 73 % were reached while the torch operated at 100 A current with 6 slpm of CH4 flow along with 15 and 20 slpm of Ar and CO2 gas flow, respectively. Torch efficiency decreased to 12 % with increase in discharge current from 100 A to 200 A. Optical emission spectrum of the plasma jet was recorded at the exit of the torch nozzle to understand the emission characteristics of the plasma and estimate temperature and electron density. Temperature of the plasma increased from 13,400 to 17,800 K with respect to the discharge current. Thermodynamical studies were carried out for the given experimental conditions to understand the thermal decomposition of CO2-CH4 gas mixture and select the suitable condition for the material processing. Under the selective conditions, plasma torch was utilized for the spheroidization of tungsten metal powder and carburization of the AISI 316 L stainless-steel coupons in atmospheric pressure without using any shielding gases. Results showed that around 85 % of the metal powders were spheroidized perfectly at 15 kW input power. Plasma carburization results showed that the microhardness of the steel surface reached 605 HV with a short processing time of 3 min which was 4 times higher than the untreated steel surface. This work provides useful insight for the utilization of CO2 as primary plasma forming gas to generate thermal plasma for material processing.

Effects of gas flow ratio and annealing on the structure and optical band gap of Mo2C thin films

Molybdenum carbide (Mo2C) has attracted increasing attention in the fields of photocatalysis, electrocatalysis, and superconductivity due to its unique structure and electronic properties. In this work, Mo2C thin films were prepared by RF magnetron sputtering to investigate the effects of CH4 flow rate and post-annealing on the structure and properties of the thin films. Results show that the grains of the prepared films are uniform and gradually decrease with the increase in CH4 flow rate. Face-centered cubic Mo2C exists in the films and preferentially grows from (002) crystal planes parallel to the substrate. After annealing and recrystallization at 850 °C, β-Mo2C mainly exists in these films. The prepared Mo2C thin films all have semiconductor properties. The optical band gap of Mo2C films increases first and then decreases with the increase in CH4 flow rate. Meanwhile, the luminescence intensity of Mo2C films increases first and then decreases with the rise in CH4 flow. The analysis of the first principles calculation shows that this performance is due to the different etching degrees of Mo2C films with different CH4 flow rates, the thermal vibration of the gap C atom, and the change in the ratio of C sp2/sp3.

Effects of Carbon Doping and DC Bias Voltage on Microstructure and Mechanical Properties of AlCrCN Films Synthesized via HiPIMS.

This work compares the hardness and adhesion properties of AlCrN and AlCrCN hard coatings synthesized via HiPIMS using Al70Cr30 and Cr targets. The hardness and adhesion properties of AlCrCN films were optimized by performing deposition under various C2H2 flow rates (5, 8, 10, 13, 15, or 20 sccm) and DC bias voltages (−40, −60, −80, −100, or −120 V). EPMA results clearly indicated that the carbon content was increased from 1.9 to 12.2 at.% with increasing C2H2 flow rate from 5 to 20 sccm. XPS results confirmed a various content of chemical bonds (Cr-N, C-N, sp2, and sp3) with various C2H2 flow rate. Grain and columnar refinement in AlCrCN were derived from XRD, TEM, and SAED results. The higher hardness (28.6 GPa) and Young’s modulus (358 GPa) were obtained using an C2H2 flow rate of 5 sccm and a bias voltage of −60 V. Both of which subsequently decreased to 13.5 GPa and 212 GPa, respectively. This can be attributed to the C-N bond inhibiting the development of metal-N bonds. Increasing the bias voltage to −120 V increased the hardness to 32.9 GPa and the Young’s modulus to 372 GPa. Note that the application of bias voltage to enhance hardness should also be applicable to carbon-doped AlCrN films as well. All samples presented good adhesion characteristics (class 1; ISO26443:2008-06).